# Common forces

When you push or pull an object you apply a force on that item. Forces are commonly categorized relating to their source, how they are transmitted, or their effects. In this page, we will discuss some of the most common forces. These forces are usually responsible for the motion of the objects as expressed in the Newton's Laws of Motion. External forces are any outside forces that act on a body. A free-body diagram is a drawing of all external forces acting on a body and when the forces on a system are balanced then the system is in equilibrium. The SI unit of force is the newton (N), N = kg·m/s2. newton equal kilograms per meter divided by seconds to the square

## Normal Force

The normal force is the force a solid object exerts in opposition to being passed through. It is called normal because it is perpendicular to the surface of the object. It is generally denoted by N N .

An example of a normal force is a situation in which an object stands on a table. The weight of the object pulls it down through the table, but the table exerts an upward normal force that balances the weight of the object.

## Weight

The force acting on an object due to gravitation is called weight.

## Friction

Friction is a resistive force opposing the motion or the tendency of an object to move.

## Spring force

A spring is a body that has the ability to restore its shape if deformed. To restore its shape, a spring exerts a force that is proportional to its displacement from the equilibrium position and in the opposite direction of said displacement. This is the statement of a law known as Hooke’s law.

Figure 1: Common forces applied to an object in motion (FG is the weight, fk is the kinetic friction, N is the normal force and F is the pushing force)